The invention relates generally to semiconductor device and integrated circuit fabrication and, in particular, to device structures and fabrication methods for a heterojunction bipolar transistor.
A bipolar junction transistor is a three-terminal electronic device that includes an emitter, a collector, and an intrinsic base arranged between the emitter and collector. In a PNP bipolar junction transistor, the emitter and collector are composed of p-type semiconductor material, and the intrinsic base is composed of n-type semiconductor material. In an NPN bipolar junction transistor, the emitter and collector are composed of n-type semiconductor material, and the intrinsic base is composed of p-type semiconductor material. When powered during operation, the base-emitter junction is forward biased, the base-collector junction is reverse biased, and the collector-emitter current may be controlled by a base-emitter voltage.
A heterojunction bipolar transistor is a variant of a bipolar junction transistor in which at least two of the collector, emitter, and intrinsic base in the device structure are constituted by semiconductor materials with different energy bandgaps, which creates heterojunctions. For example, the collector and/or emitter of a heterojunction bipolar transistor may be constituted by silicon, and the base of a heterojunction bipolar transistor may be constituted by silicon germanium (SiGe), which is characterized by a narrower band gap than silicon.
Improved device structures and fabrication methods for a heterojunction bipolar transistor are needed.